Folding wall type beds, desks, tables and other pieces of furniture are widely used in situations in which available space is at a premium. The bed or other piece of furniture is provided with hinges so that it can be pivoted upwards from its generally horizontal use position, to a generally vertical storage position adjacent a wall, or in a small closet placed in a wall for that purpose.
Because the weight involved in even a medium sized bed can be considerable, it has long been considered desirable to provide some type of counterbalance springs to make it easier for a person to move the bed between the in use and storage positions. It has also been long recognized that the spring bias applied as a torque about the bed pivot to oppose the weight of the bed is not a linear function of the position of the bed, but is in the nature of a sine function of the angular position of the bed. This of course is because the effective lever arm of the center of gravity of the bed about the pivot point increases sinusoidally as the bed is brought down from the vertical to the horizontal position.
The same situation exists with respect to all types of pivoting or fold up loads, including but not limited to, fold up desks, tables, work counters, loading ramps or doors hinged at the bottom, or any other member pivoted near its bottom for movement between a generally vertical to a generally horizontal position. For purposes of illustration, the present invention as disclosed herein is applied to a folding wall type bed, but it will be understood that the present invention is equally applicable to any of the pivoting type loads discussed above.
Numerous arrangements have been proposed in the prior art to match the essentially linear response of a spring to the inherently nonlinear counterbalancing force requirements of a fold up bed. One such prior art arrangement uses torsion bar springs as the main counterbalance for the bed. Additional springs and linkages are then used to counter the undesired effects of the torsion bars when the bed is near the horizontal, use position. This prior art structure involves the disadvantages of nonuniformity among manufactured torsion bars, limited range of adjustment to compensate for the nonuniformities and also for variations in bed weight, and a torsion bar breakage problem. This prior art structure also has the disadvantage of excessive cost and complexity, due to the necessity of the compensating springs and linkages.
Other prior art structures have proposed the use of a specially shaped cam to modify the force of the counterbalancing spring, as a function of the angular position of the bed. A spring is attached to one portion of the bed hinge or pivot and to the other portion of the hinge by means of a cable, belt or rope which passes around a cam surface which is fixed to move with the pivoting of the bed. In these prior art devices, the radius of the cam increases as the bed moves from the vertical to the horizontal position, because the portion of the weight of the bed to be overcome by lifting is greatest when the bed is near its horizontal position, diminishing to zero when the bed is in its vertical position. It was apparently thought that the large radius would give the spring the necessary leverage to handle the weight of the bed at its horizontal position, and that less leverage was needed as the bed approached the vertical position.
While the above theory for the shape of the cam appears reasonable at first glance, in actual practice we have found the opposite to be true; namely that the effective radius with the bed in its horizontal position should be less than the effective radius with the bed in its vertical position.
The reason for this seeming contradiction is that the prior art structures referred to above fail to take into account the effect on the degree of tension or compression of the spring due to the shape of the cam itself. It is thus necessary to consider not only the effective radius of the cam at a given point, but the tension or compression of the spring at that same point, which of course determines the force applied by the spring. But the compression or tension of the spring is itself a function of the total path length or peripheral length over the surface of the cam from the start up to the point in question. It is this path length factor which was apparently overlooked in the prior art devices discussed above.
Thus, the problem is not merely one of multiplying the spring force, assumed to be more or less constant, by the variable effective radius of the cam. Instead, if excessively long springs are to be avoided, which would lead to greater complexity, expense, and space requirements, it must be recognized that not only is the effective lever arm of the cam a function of its angular position, but the spring displacement and hence force developed is also a function of the angular position of the cam, since the displacement of the spring is determined by the peripheral length around the surface of the cam.